Internal-combustion engines fall into four categories, defined by the amount of mixing in the air/fuel charge and how the charge is ignited. In a standard gasoline engine air and fuel are premixed into a well-mixed charge that is ignited with a spark. In a standard diesel engine fuel is sprayed into the cylinder during the piston's compression stroke. Air and fuel are not well mixed when the rising temperature from compression induces burning. In the homogenous charge compression ignition (HCCI) engine air and fuel are well mixed before self-ignition due to compression. In an HCCI engine the air and fuel can be premixed as in a gasoline engine or fuel can be directly injected like a diesel but earlier to allow better air/fuel mixing before ignition. Better air/fuel mixing produces much lower NOx and PM emissions compared to standard diesel combustion where rich fuel/air pockets lead to soot formation, and flame surrounding the injected fuel leads to high NOx. HCCI has better fuel economy than a gasoline engine because there is no throttling, and because higher compression ratios can be used.
The main challenges for HCCI are both too rapid combustion, and lack of a triggering ignition event. Once the temperature in the HCCI engine cylinder is sufficiently high, the premixed air/fuel mixture combusts rapidly. If combustion is too rapid then high pressure rise rates can occur causing excessive noise and potential engine damage. The lack of an ignition triggering event makes it more difficult to control an HCCI engine. In a gasoline engine a spark triggers ignition, while in a diesel engine fuel injected into hot compressed air triggers ignition. These measures cannot be used to control ignition timing and duration in an HCCI engine.
Another challenge for HCCI engines is their limited load range caused by the use of high excess air/fuel ratios and/or high EGR rates to control combustion phasing, cylinder pressure, rate of cylinder pressure rise and/or NOx emissions.
Currently, some of the challenges facing HCCI engines include:
Limited load range;
Lack of universal, yet practical, measures of ignition quality of HCCI fuels; and
Excessive particulate/smoke emissions during operation on diesel boiling range fuels, especially at high engine loads.
HCCI technology is still at a relatively early stage of development, yet holds great promise due to its excellent exhaust emission and fuel efficiency characteristics. Different versions/configurations of HCCI engines are being developed worldwide. Their commercialization has been held back mainly by the challenges of this technology, including limited load range, difficulties in controlling combustion phasing and excessive heat release rates. In order to address some of these challenges and opportunities related to HCCI, Caterpillar has entered into a cooperative research and development agreement with ExxonMobil Corporation, since the Fall of 2002. This disclosure is a result of their joint work.
In the following disclosure, diesel fuel is defined as a mixture of hydrocarbons which boil at atmospheric pressure over a temperature range within about 150° C. to 0.380° C., whereas gasoline is defined as a mixture of hydrocarbons which boil at atmospheric pressure over a temperature range within about 25° C. to 220° C.
HCCI engines are operated at high air/fuel ratios and/or high EGR rates for the purpose of controlling combustion phasing, peak cylinder pressure, rate of cylinder pressure rise and/or NOx emissions. This restricts the amount of fuel that can be burned in the course of an engine cycle and thus limits the maximum achievable engine loads. For example, HCCI engines operated on a typical 45 cetane number US diesel fuel can produce, at most, only ⅓ of the load attainable by comparable diesel engines, if the comparison is made at the same diesel-like compression ratio.